Automatic Teller Machines (ATMs) are provided with several types of sensors in order to perform various kinds of measurements against the inserted bank notes, thereby distinguishing the type of notes or checking for fake notes.
One of the measurements performed against the inserted bank notes by the ATM is to measure the thickness of the bank notes. By measuring the thickness, an overlapping of two or more bank notes due to an abnormal conveyance can be detected, or whether or not the bank notes are normal bank notes can be determined. Moreover, the existence of a foreign object on the paper can be checked by detecting the local distribution of the thickness.
Generally, methods of holding the paper by pressurizing it with two rollers and measuring the gap between the rollers are well-known as methods of measuring the thickness of pieces of paper. The thickness of pieces of paper can reliably be measured by using this method, but paper jamming is easily caused when the measurement is performed with pieces of paper being fed at a high speed, which is a problem.
In response to that problem, electrostatic capacitance thickness detecting methods that can measure the thickness without mechanically pressurizing the paper are becoming widespread. These electrostatic capacitance thickness detecting methods are for detecting the thickness of pieces of paper by using the change in electrostatic capacitance between the counter electrodes when the pieces of paper are inserted between the opposing electrodes. Although the change in the electrostatic capacitance depends on the thickness of the pieces of paper and the size of the relative permittivity in these methods, the thickness of the pieces of paper can be known from the change in the electrostatic capacitance as the size of the relative permittivity of the pieces of paper to be measured is constant.
A paper thickness detecting apparatus 100 of FIG. 1 is a general electrostatic capacitance paper thickness detecting apparatus. The paper thickness detecting apparatus 100 is provided with an electrostatic capacitance sensor (hereinafter, this sensor is referred to as “sensor array”) 101 comprised of counter electrodes 101a-101e, and a thickness detector 102 for detecting the thickness of a piece of paper from response signals in response to signals input into the sensor array 101 (, the input signals are common for each of the counter electrodes 101a-101e).
Each of the counter electrodes 101a-101e constitutes a capacitor with a predetermined electrostatic capacitance. The thickness detector 102 receives response signals in response to common input signals (for example, sinusoidal signals) that are input to the counter electrodes 101a-101e. Then, the thickness of a piece of paper 103 is detected from those response signals. The common input signals to be provided to the sensor array 101 are input from a signal source (not illustrated) to the sensor array 101.
When the paper 103 is conveyed in a conveyance direction X and is inserted between the electrodes of the respective counter electrodes 101a-101e, the electrostatic capacitance of the counter electrodes 101a-101e changes, as described above. At this time, the response signals change as illustrated in FIG. 2A and FIG. 2B.
FIG. 2A and FIG. 2B are diagrams illustrating exemplary response signals when the paper 103 is inserted between the electrodes of the respective counter electrodes 101a-101e. The horizontal axis of FIG. 2A and FIG. 2B represents the counter electrodes 101a, 101b, . . . , and 101e (hereinafter, they are referred to as “detection channel” as a whole), and the vertical axis represents the signal (electric current) detected by the detection channel.
As illustrated in FIG. 2A, when the paper 103 is inserted between the electrodes of the counter electrodes 101a-101e, the signal of the counter electrode at which there is a fold on the paper increases sharply (for example, “a” illustrated in FIG. 2A).
However, the electrostatic capacitance of the counter electrodes 101a-101e also changes when a foreign object is adhered to the paper, and thereby similar response signals as in FIG. 2A are obtained, as illustrated in “b” of FIG. 2B.
As described in the above, there has been a problem in which it is difficult to detect only a foreign object adhered to the paper as it is not possible to determine whether the change in the response signals is caused by folds in the paper or by a foreign object such as a piece of tape adhered to the paper.
In relation to the above-mentioned technique, Patent Document 1 discloses a paper thickness abnormality detection apparatus capable of detecting a thickness abnormality even when pieces of paper are fed at a high speed by performing a thickness abnormality detection with a detector having two or more condensers, and of speedily and accurately performing a detecting operation.
Patent Document 2 discloses a paper discrimination apparatus capable of reforming the nonuniform electric field between electrodes by rounding or chamfering the end face of the electrodes on the sides where pieces of paper are conveyed and by covering the electrodes with dielectric material, and of reducing the change in electrostatic capacitance between the electrodes due to the up-down fluctuation of the paper when the paper is conveyed between the electrodes.
Patent Document 3 discloses a paper discrimination sensor for discriminating the state of pieces of paper conveyed through a conveyance path by using the change in electrostatic capacitance between the counter electrodes arranged in the conveyance path of the paper, where a piece of tape or a seal can accurately and reliably be discriminated by forming the sensor body with the electrodes embedded onto the dielectric material and by providing a layer of conductive material whose surface resistance value is 104-109Ω on the surface of the sensor body on the sides where the paper pass through.
Patent Document 1: Japanese Patent Application No. H 02-098605
Patent Document 2: Japanese Patent Application No. 2001-240271
Patent Document 3: Japanese Patent Application No. 2004-280367